The invention relates to a method for detecting disturbances when determining the rotational speed of a rotor and to an evaluation circuit for a rotary encoder. Rotary encoders are used to determine the rotational speed, the direction of rotation and the angle of rotation of a rotor with respect to a sensor arrangement. Rotary encoders of this type are used in a wide variety of industrial sectors, in particular in drive technology
FIG. 1 shows a rotor 1, which is designed for example as a metallic gearwheel with teeth 2, of a rotary encoder. A sensor arrangement comprising sensors 31, 32 and 33 is situated at a distance d from the rotor 1. Said sensors 31, 32, 33 may be designed for example as Hall sensors or as inductive proximity switches.
As a result of a rotation of the rotor 1 about its shaft 3, the sensor arrangement generates a phase signal on account of the teeth 2 of the rotor 1 moving past the sensors 31, 32, 33. In the case of a uniform undisturbed rotation of the rotor, each of the sensors 31, 32, 33 supplies a periodic phase signal, which, depending on the concrete configuration of the sensor, may be a sinusoidal signal or a square-wave signal. An item of information about the rotational speed of the rotor can be inferred directly from said phase signals by determining the zero crossings, in which case the number of teeth of the wheel is to be taken into account.
A possible procedure for generating a first rotational speed signal, which contains an item of rotational speed information, and a first direction of rotation signal, which contains an item of direction of rotation information, is illustrated in FIG. 2. An arrangement in accordance with FIG. 2 is described in DE 197 17 364 C1. The sensors 31, 32, 33 respectively represent a first, a second and a third signal source, which respectively output a phase signal S3, S4 and S5. The outputs 315 and 335 of the first and third signal sources 31 and 33, respectively, are connected to the first input 101 and the second input 102, respectively, of a first subtractor 10, which forms the difference between the first phase signal S3 and the third phase signal S5 and outputs this difference as a first rotational speed signal S1 at its output 105.
The first phase signal S3 and the third phase signal S5 are formed for example as sinusoidal signal or as square-wave signal with a duty ratio of 1:1, and have a phase difference 180° and also the same amplitude. In this case, the first rotational speed signal S1 is likewise formed in sinusoidal or square-wave fashion, the amplitude being doubled compared with the first or third phase signal S3 or S5, respectively.
In order to generate a first direction of rotation signal S2, firstly the first phase signal S3 and the third phase signal S5 are fed to the first and second inputs 111 and 112, respectively, of a first adder 11. The signal present at the output 115 thereof is passed to the input 141 of an amplifier 14, which multiplies the amplitude of the signal by a factor of 0.5 and outputs the thereby attenuated signal at its output 145. A second subtractor 12 forms the difference between said attenuated signal and the second phase signal S4 output by the second signal source 32. The corresponding difference signal is output at the output 125 of the second subtractor 12 and forms the first direction of rotation signal S2.
In the case where the signal sources 31, 32, 33 are suitably laterally spaced apart with respect to the teeth 2 of the rotor 1 illustrated in FIG. 1, the first rotational speed signal S1 and the first direction of rotation signal S2 have a phase difference of +90° or −90°, it being possible, in principle, to set arbitrary phase differences. In this case, the sign of this phase difference depends on the direction of rotation of the rotor 1 shown in FIG. 1.
A typical temporal profile of a first rotational speed signal S1 and of a first direction of rotation signal S2 is illustrated schematically in FIG. 3. Both signals are sinusoidal, the amplitude of the first rotational speed signal S1 being twice as large as the amplitude of the first direction of rotation signal S2.
The two signals S1 and S2 have a phase difference Δφ1. In the example explained, the phase difference Δφ1 is ascertained by means of the distance between the zero crossings of the first rotational speed signal S1 at a first instant t1 and the succeeding zero crossing of the first direction of rotation signal S2, in each case with a falling signal. A further possibility for ascertaining the phase difference Δφ1 consists e.g. in using a zero crossing of the first rotational speed signal S1 as first instant t1, determining the amplitude of the first direction of rotation signal S2 at this instant t1 and calculating the phase difference Δφ1 by means of a known correlation between amplitude and phase of the direction of rotation signal S2.
Disturbances may be superposed on the first rotational speed signal S1 and the first direction of rotation signal S2, which disturbances can adversely influence the measurement result. Disturbances of this type include for example so-called distance oscillations, as a result of which the distance d between the rotor 1 and the sensor arrangement 31, 32, 33 changes, or rotary oscillations of the rotor, by means of which the rotor 1 oscillates about its instantaneous angular position during the rotation about its shaft 3. Disturbances of this type alter the phase signals output by the sensors 31, 32, 33.
As long as the rotor rotates uniformly in a direction of rotation and as long as there is no disturbance present, the phase difference between the first rotational speed signal S1 and the first direction of rotation signal S2 is constant. A change in the rotational speed or the direction of rotation gives rise firstly to a temporal change in the phase difference, which is generally small in comparison with the phase differences caused by relevant disturbances, so that in general a differentiation is possible.
WO 03/098229 A1 discloses a method and an arrangement for ascertaining rotational speed, in which the switching hysteresis is adapted dynamically in a manner dependent on the distance between the sensor arrangement and the rotor.
WO 03/098230 A1 shows a method and an arrangement for detecting the movement of an element, in which the pulses output by a sensor arrangement are summed in a direction-dependent manner by means of an up/down counter. If the sensor arrangement generates counting pul-pulses brought about by a vibration, then said pulses are essentially averaged to a counter value close to zero.
WO 03/100352 A1 discloses a method and an arrangement for detecting the movement of an element relative to a sensor arrangement, in which the phase responses of individual sensor elements and of the difference signal of the individual sensor elements, for identifying vibrations, are evaluated in respect of whether all three signals are in phase.